Norbornadienes for Solar Thermal Energy Storage and New Applications

Doctoral thesis, 2019

The energy demand worldwide is steadily increasing, therefore it is fundamental to efficiently utilise renewable energy resources. Energy storage technologies are partic- ularly relevant in order to be able to exploit renewable energy resources such as solar energy, since these are typically intermittent and not evenly distributed. The work presen- ted in this thesis is focused on trying to optimise norbornadiene-quadricyclane systems to harness and store solar energy. Norbornadienes are able to absorb light, and undergo photoinduced isomerization to the high energy isomer quadricyclane, which is stable over time. When quadricyclanes back isomerise to norbornadienes they release the absorbed energy as heat. This technology is called “molecular solar thermal”, or MOST. Differ- ent features need to be optimised in order to utilise norbornadiene-quadricyclane pho- toswitches for MOST applications. In my work I focused on synthesising new norbor- nadienes, characterising their molecular and spectroscopic properties, and trying to op- timise them for energy storage purposes. In particular I focused on identifying specific structure-properties relationship that allow selectively engineering the kinetic stability of quadricyclanes, to achieve longer storage times. This was in fact achieved in a series of norbornadienes by selectively increasing the entropy of activation to the back isomeriza- tion. A small device was also built, in order to test a hybrid technology that would combine MOST and solar water heating. These laboratory-scale experiments were particularly in- structive in demonstrating the potential of MOST systems, and learning about the future challenges. Liquid, neat norbornadienes were also made, and their properties assessed. They retained the ability to photoisomerise and back convert in neat samples, but new challenges arose, such as stability over multiple cycles and storage times. Moreover, the use of a norbornadiene-quadriyclane photoswitch as a molecular keypad lock is demon- strated.

The energy demand worldwide is steadily increasing, which is impacting dramatically the global environment. It is fundamental to efficiently utilise renewable energy resources. Energy storage technologies are particularly relevant in order to be able to exploit renewable energy resources such as solar and wind energy, since these are typically intermittent and not evenly distributed. The work presented in this thesis is focused on trying to optimise the organic molecular switch norbornadiene-quadricyclane to harness and store solar energy. Norbornadienes are able to absorb light, and when they do it they change to the high energy quadricyclanes, which are stable over time. When quadricyclanes back isomerise to norbornadienes they release the absorbed energy as heat. This technology is called “molecular solar thermal”, or MOST. In my work I focused on synthesising new norbornadienes, characterising their properties, and trying to optimise them for energy storage purposes. In particular I focused on identifying ways to achieve longer storage times. This was in fact achieved in a series of norbornadienes by selectively increasing a parameter called entropy of activation. A small device was also built, in order to test a hybrid technology that would combine MOST and solar water heating. Testing a norbornadiene in a real device was particularly instructive in demonstrating the potential of MOST systems, and learning about the future challenges. For the first time liquid MOST materials based on norbornadienes were made, which are important for MOST technologies based on circulation of liquids. Moreover, the use of a norbornadiene-quadriyclane photoswitch as a molecular keypad lock (where only a unique set of inputs can give a specific output) was demonstrated.